fork() creates a new process by duplicating the calling process. The
new process is referred to as the child process. The calling process
is referred to as the parent process.
The child process and the parent process run in separate memory
spaces. At the time of fork() both memory spaces have the same
content. Memory writes, file mappings (mmap(2)), and unmappings
(munmap(2)) performed by one of the processes do not affect the
other.
The child process is an exact duplicate of the parent process except
for the following points:
* The child has its own unique process ID, and this PID does not
match the ID of any existing process group (setpgid(2)) or
session.
* The child's parent process ID is the same as the parent's process
ID.
* The child does not inherit its parent's memory locks (mlock(2),
mlockall(2)).
* Process resource utilizations (getrusage(2)) and CPU time counters
(times(2)) are reset to zero in the child.
* The child's set of pending signals is initially empty
(sigpending(2)).
* The child does not inherit semaphore adjustments from its parent
(semop(2)).
* The child does not inherit process-associated record locks from
its parent (fcntl(2)). (On the other hand, it does inherit
fcntl(2) open file description locks and flock(2) locks from its
parent.)
* The child does not inherit timers from its parent (setitimer(2),
alarm(2), timer_create(2)).
* The child does not inherit outstanding asynchronous I/O operations
from its parent (aio_read(3), aio_write(3)), nor does it inherit
any asynchronous I/O contexts from its parent (see io_setup(2)).
The process attributes in the preceding list are all specified in
POSIX.1. The parent and child also differ with respect to the
following Linux-specific process attributes:
* The child does not inherit directory change notifications
(dnotify) from its parent (see the description of F_NOTIFY in
fcntl(2)).
* The prctl(2)PR_SET_PDEATHSIG setting is reset so that the child
does not receive a signal when its parent terminates.
* The default timer slack value is set to the parent's current timer
slack value. See the description of PR_SET_TIMERSLACK in
prctl(2).
* Memory mappings that have been marked with the madvise(2)MADV_DONTFORK flag are not inherited across a fork().
* Memory in address ranges that have been marked with the madvise(2)MADV_WIPEONFORK flag is zeroed in the child after a fork(). (The
MADV_WIPEONFORK setting remains in place for those address ranges
in the child.)
* The termination signal of the child is always SIGCHLD (see
clone(2)).
* The port access permission bits set by ioperm(2) are not inherited
by the child; the child must turn on any bits that it requires
using ioperm(2).
Note the following further points:
* The child process is created with a single thread—the one that
called fork(). The entire virtual address space of the parent is
replicated in the child, including the states of mutexes,
condition variables, and other pthreads objects; the use of
pthread_atfork(3) may be helpful for dealing with problems that
this can cause.
* After a fork() in a multithreaded program, the child can safely
call only async-signal-safe functions (see signal-safety(7)) until
such time as it calls execve(2).
* The child inherits copies of the parent's set of open file
descriptors. Each file descriptor in the child refers to the same
open file description (see open(2)) as the corresponding file
descriptor in the parent. This means that the two file
descriptors share open file status flags, file offset, and signal-
driven I/O attributes (see the description of F_SETOWN and
F_SETSIG in fcntl(2)).
* The child inherits copies of the parent's set of open message
queue descriptors (see mq_overview(7)). Each file descriptor in
the child refers to the same open message queue description as the
corresponding file descriptor in the parent. This means that the
two file descriptors share the same flags (mq_flags).
* The child inherits copies of the parent's set of open directory
streams (see opendir(3)). POSIX.1 says that the corresponding
directory streams in the parent and child may share the directory
stream positioning; on Linux/glibc they do not.

On success, the PID of the child process is returned in the parent,
and 0 is returned in the child. On failure, -1 is returned in the
parent, no child process is created, and errno is set appropriately.

EAGAIN A system-imposed limit on the number of threads was
encountered. There are a number of limits that may trigger
this error:
* the RLIMIT_NPROC soft resource limit (set via
setrlimit(2)), which limits the number of processes and
threads for a real user ID, was reached;
* the kernel's system-wide limit on the number of processes
and threads, /proc/sys/kernel/threads-max, was reached (see
proc(5));
* the maximum number of PIDs, /proc/sys/kernel/pid_max, was
reached (see proc(5)); or
* the PID limit (pids.max) imposed by the cgroup "process
number" (PIDs) controller was reached.
EAGAIN The caller is operating under the SCHED_DEADLINE scheduling
policy and does not have the reset-on-fork flag set. See
sched(7).
ENOMEM fork() failed to allocate the necessary kernel structures
because memory is tight.
ENOMEM An attempt was made to create a child process in a PID
namespace whose "init" process has terminated. See
pid_namespaces(7).
ENOSYS fork() is not supported on this platform (for example,
hardware without a Memory-Management Unit).
ERESTARTNOINTR (since Linux 2.6.17)
System call was interrupted by a signal and will be restarted.
(This can be seen only during a trace.)

Under Linux, fork() is implemented using copy-on-write pages, so the
only penalty that it incurs is the time and memory required to
duplicate the parent's page tables, and to create a unique task
structure for the child.
C library/kernel differences
Since version 2.3.3, rather than invoking the kernel's fork() system
call, the glibc fork() wrapper that is provided as part of the NPTL
threading implementation invokes clone(2) with flags that provide the
same effect as the traditional system call. (A call to fork() is
equivalent to a call to clone(2) specifying flags as just SIGCHLD.)
The glibc wrapper invokes any fork handlers that have been
established using pthread_atfork(3).

This page is part of release 5.01 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 FORK(2)